Merge branch '2023-10-06-spl-prepare-for-universal-payload'

To quote the author:
This series tidies up SPL a little and adds some core ofnode functions
needed to support Universal Payload. It also includes a few minor
fix-ups for sandbox.

For SPL the changes include CONFIG naming, removing various #ifdefs and
tidying up the FIT code.

One notable piece of the ofnode improvements is support for flattening a
livetree. This should be useful in future as we move FDT fixups to use
the ofnode API.